Tuberculosis (TB) is a persistent lung infection that has plagued mankind for centuries and ranks as one of the most serious threats to world health today. The 2-3 million deaths attributed yearly to the disease, as well as the emergence of strains resistant to all of the available chemotherapeutic agents, urgently call for the development of new therapies to treat TB. For years, the identification of new drug targets has been hampered by the intractability of the bacillus to genetic analysis. Now with the advent of powerful genetic tools, combined with well-established mouse infection models, we have isolated novel M. tuberculosis mutants with lesions in individual genes that are required for normal growth during acute infection. Our results have led us to the hypothesis that M. tuberculosis influences host-pathogen contacts by utilizing two virulence secretion systems: the MmpL family of transporters, which secretes polyketide lipids to the bacterial cell surface, and the Snm secretion system, which secretes proteins into host cells. Both of these secretion pathways are central to M. tuberculosis virulence. The studies proposed here will seek to determine the protein complexes that target and translocate virulence factors across the cell membrane in M. tuberculosis. In the case of MmpL proteins, we will test the hypothesis that interactions between synthase and transporter provide efficiency and specificity to lipid transport. Likewise, we will use our knowledge of some of the proteins that make up the Snm secretion system to identify new components using both genetics and biochemistry. The results from these studies will direct our long-term plans to understand the mechanism by which M. tuberculosis virulence factors are secreted from the bacterium and into the host. Ultimately, by understanding tuberculosis pathogenesis at the molecular level, we hope to aid in the discovery of new therapies to combat and eradicate this persistent infection.